In one prior art, a sensor is installed in a rod body of a ratchet wrench. The sensor includes a digital display, a strain gauge and a signal amplifier. The deformation of the strain gauge generates a variation of resistance so as to derive the twisting force applied thereon. For this arrangement, the rod body must be formed with a through hole or a groove for installing the sensor. Then the rod body is refinished so that the cost is high.
Moreover, since the strain gauge is installed within the rod body, it is inseparable. Thereby for different hand tool, a different strain gauge is necessary and thus the cost is increased.